The rapid, uniform and gentle controlling of the temperature of viscous and highly viscous products, e.g. polymer melts, is only achieved to an insufficient extent using the known static mixer systems described below. Only the outer temperature-controlled housing or tube wall is available as a direct heating surface for these purposes. To control the temperature of a product, the latter is passed a number of times through the known static mixers from the center of the housing or tube to the temperature-controlled housing wall, so that the desired product temperature is reached over an increasing length of the heating section. Temperature-control objectives of this type require long temperature-controlled mixing distances, on account of the low thermal conductivity of most organic substances, leading to a long residence time and a high pressure loss and therefore to damage to viscous substances (>1 mPa·s) with a laminar flow velocity, in particular those with a temperature-sensitive character. An additional drawback of the long mixing distances is the high design-related investment costs involved with such systems. Drawbacks such as the low mechanical stability and high pressure losses of known static mixers lead to the need for large cross sections of flow, which in turn make temperature control more difficult.
A slight improvement in terms of temperature-control objectives is achieved if known static mixers are pressed or rolled into pipelines or into housings. This results in limited metallic contact between the heated inner housing wall and the small outer cross-sectional areas of the metallic static mixers. However, the static mixer which has been drawn or rolled in can only form an inadequate contact surface with the temperature-controlled housing wall. Experience has shown that the contact surfaces are not formed completely, and consequently there are always gaps with respect to the inner housing wall. On account of higher thermal conduction properties of the metallic mixing fins, small amounts of heat are passed radially through these narrow gaps into the flow region of the static mixer. This method allows a slight improvement only with very small housing or tube diameters, since the conduction of heat to the center of the static mixer or the housing is limited by the small, incompletely formed contact surfaces. Furthermore, these gaps represent “dead areas”, which contribute to the formation of specks, for example in polymer melts. These specks (impurities) reduce the quality of the products sold (e.g. thermoplastics).
Known static mixers which are soldered into housings or pipelines have slightly better temperature-control properties. The soldering operation requires an accurately prepared housing or pipe and a static mixer which has been machined on its external diameter, so that a good and complete soldered joint can be produced. The mechanical preparations which have to be carried out on the parts to be soldered are complex and cost-intensive. If soldering is successful, static mixers which are soldered in have a good contact surface with respect to the inner temperature-controlled housing wall. On account of the geometric structure of the static mixers, however, the contact surface with respect to the heated housing surface is very small, and consequently only a slightly higher temperature-control capacity with respect to the product flow is possible. The increase in the size of the temperature-controlled surface area compared to the static mixers which are rolled in is not significantly higher, and consequently mixing distances with soldered static mixers cannot be shortened significantly. On account of the limited overall size of soldering furnaces and on account of the distortion caused to the tubes during soldering, the soldering process is only possible for a short length of tube (generally <2 m).
Moreover, the solder used means that additional corrosion problems often occur and have to be taken into account during use of mixers of this type, in order to ensure that, for example, the purity and quality of a product are not adversely affected by impurities resulting from corrosion.
Furthermore, tubes with outer thin sheet metal discs which have been drawn on, pressed in or attached by welding are known for heat transfer with liquid and gaseous substances. The outer thin discs are not completely in contact with the actual carrier tube, and consequently they are preferably used to control the temperature of air in the highly turbulent flow region. These designs are not pressure-stable and do not have any mixing properties for viscous substances in the laminar flow region. Therefore, tube systems of this type are not suitable for controlling the temperature of viscous and highly viscous liquids. To improve the heat-transfer properties, by way of example, these outer discs and the carrier tube are completely covered with a low-temperature solder in order to increase the size of surfaces which are in contact with product and thereby to increase the heat conduction. The solders used (e.g. zinc, tin) cannot be used in chemical processes with high corrosion specifications, and furthermore the mechanical strength of solders of this type is very low, in particular in the event of high thermal loads.
Furthermore, a temperature-controllable static mixer reactor (DE 2 839 564 A1) is known. This reactor mixes the product flowing through, the mixing internals comprising meandering tubes. This apparatus comprises a housing, the temperature of which can be controlled and in which the mixing internals are replaced by a specially shaped meandering tube bundle.
The tube bundle comprises a plurality of bent, thin tubes running parallel to one another. The ends of the tubes are welded to a flange, from which the heating or cooling agent for controlling the temperature of the product stream is fed in.
The bent tubes running parallel to one another are fitted into the housing, parallel to the direction of flow of the product, as temperature-controlled internals. The meandering tubes are positioned at an alternating angle in the direction of flow of the product and run transversely over the hydraulic diameter of the housing. The tubes arranged in parallel in the bundle cross one another in the axial direction of the housing, in accordance with the known static mixer principle. In this design, the mixing tubes have a round to elliptical flow-facing cross section, and the tubes are inclined at an angle with respect to the product flow, so that there is only a slight distributing diversion or mixing of the product flow whose temperature is to be controlled. Since flow-facing round profiles have a low mixing action, a homogeneous temperature distribution in a high-viscosity product flow cannot be achieved to a sufficient extent over a short distance.
The length of the meandering tube bundle which can be plugged in is always a multiple of the housing hydraulic diameter. On account of their elongated length, the meandering bent tubes have a large heat-transfer surface area. The liquid heat-transfer medium, which releases its energy via the tube bundle around which the product flows, is supplied and discharged through the connecting flange. Particularly when the temperature of viscous substances, which have heat-insulating properties, is being controlled, the large heating surface area cannot be utilized effectively, since the internals do not have a good mixing action.
The bent plug-in tube bundles are susceptible to large pressure gradients. During starting-up operations or in the event of a product blockage caused by highly viscous products, high pressure gradients occur, and consequently the meandering bent heating/cooling tubes are subjected to tensile or compressive loads in the direction of flow of the product and are stretched. The inner heat-transfer internals of the apparatus tend to be deformed in the process, and further control of the temperature of the product is then no longer possible, on account of the absence of diversion of the product. The undesired stretching of the tube bundle is irreparable and may lead to the plant having to be shut down, with high downtime costs.
On account of the ideally elongated length of the individual tube and the small cross section of flow, the temperature-controllable meandering tube bundle has a high pressure loss and a long residence time on the temperature-control side. The combination of the two, i.e. pressure loss and residence time of for example the temperature-control medium in the meandering turns, leads to considerable differences between the inlet temperature and the outlet temperature and reduces the mean temperature difference between the product and the heat transfer media, which is important for heat transfer, significantly. Consequently, the heat-transfer performance of meandering tube bundles of this type is low. In practice, a plurality of tube bundles are often connected in series, and this in turn increases the investment costs, the pressure loss, and the residence time of the substance whose temperature is to be controlled (i.e., the product) and also increases the outlay on assembly.
A uniform and gentle control of the temperature of highly viscous, single-phase or multiphase product flows combined, at the same time, with a short residence time cannot be achieved with the known systems, such as for example static mixers with heatable housings or the temperature-controllable meandering tube bundles.
A need therefore exists for a static mixer whose temperature can be controlled and which has heating passages in the product flow and good mixing properties. Such temperature-controllable static mixers are to have a low pressure loss on the heat-transfer medium side, so that it is possible to reckon on large temperature differences with respect to the temperature-controllable product flow. Furthermore, it is desirable to be able to apply such apparatus concept to large housing hydraulic diameters. An additional improvement with regard to high robustness with respect to mechanical effects, with respect to high pressure gradients and the option to use various thermally conductive and corrosion-resistant materials, in order to satisfy different product demands, would also be advantageous.
There are further demands which must be met with regard to successful adaptation in order to achieve different process-engineering objectives in terms of a low pressure losses on the side which is in contact with product and on the temperature-controlled side, a high mixing capacity, a low residence time spectrum on the product side, a large temperature-control surface area and a high heat transfer capacity. The apparatus is to have significant advantages for use with viscous to highly viscous substances (viscosity 0.001 to 20,000 Pa·s).
The mechanical stability during start-up operations or during assembly is to be increased, so that higher operational reliability can be achieved.
The desired apparatus would advantageously be in the form of a compact heat exchanger which could be installed in production facilities with a low installation outlay and low production costs.
To summarize, it is an object of the invention to provide a static mixer/heat exchanger which avoids the drawbacks of the designs known in the prior art, which allows significantly improved control of the temperature, combined with a smaller apparatus volume, reduces the production costs of the apparatus and has a higher robustness, operational reliability and service life than known heat exchangers.